氟哌酸的分子结构中4位的取代基是
A．甲基B．酮基C．乙基D．氟原子E．羧基
参考答案：B
—Physics is an important subject, isn’t it ?
—Yes. ________it well and you’ll find it useful in your daily life.
A．Learning
B．Learn
C．Learned
D．To learn
解释中心极限定理的含义？
